past perfect continuous

The important one that you must remember to make a sentence is a sentence must have SUBJECT and VERB. and the second is every tense has its unique auxiliary. in simple present we use do or does in negative and interrogative, in present continuous we use is, am, are followed by Ving, and so on.
In this eighth tense we use the auxiliary HAD BEEN then followed by Ving. so, from this we can say that tenses which has the word continuous at its name will always use Ving.
well, to understand past perfect continuous easier, look at the examples below carefully.

1. + He had been going to school.

- He had not been going to school.

? Had he been going to school?

2. + She had been climbing the mountain.

- She had not been climbing the mountain.

? Had she been climbing the mountain?

3. + They had been running to the forest.

- They had not been running to the forest.

? Had they been running to the forest?

4. + We had been listening to the music.

- We had not been listening to the music.

? Had we been listening to the music?

5. + I had been doing my homework.

- I had not been doing my homework.

? Had I been doing my homework?